The literature in the pyrolysis art frequently use terminology which is incorrect or misleading. For purposes of this application, some general understandings with respect to the terminology used herein and as used in the claims hereof will have the following meanings. "Pyrolysis" in a technical sense means the chemical decomposition or change in a material brought about by heating the material in the absence of oxygen. As a practical matter, oxygen is always present in the combustion chamber of any commercial waste treatment facility. This results because of the practical limitations in constructing a perfectly sealed furnace operated at standard atmosphere. Thus, if the furnace is operated at pressures slightly over standard atmosphere, fumes will leak from the furnace during the process. Since the fumes can be toxic, the furnace must be operated at a neutral to slightly negative pressure which thus draws air into the furnace during the operation. This means that pyrolysis in the technical sense cannot be performed. It must be noted that there are furnace constructions using double walled vessels with water jackets in the vacuum furnace art which can develop perfect seals and such furnaces could accomplish true pyrolysis. The waste applications under discussion and to which this invention relates involve economies of scale including size which prohibit this type of furnace construction. Accordingly, when pyrolysis is used in this specification, it is used in the sense that some oxygen is inherently present in the furnace chamber.
The specification will discuss the pyrolysis process along the classical lines of whether or not the thermal reactions are endothermic or exothermic. The atmosphere which is present in the furnace will then be discussed with reference to the percentage of volatiles present in that atmosphere as either being within or outside the explosive limit. That is, and is conventionally known (for example see assignee's U.S. Pat. No. 3,909,953), a gaseous mixture composed entirely of volatiles will ignite, combust or explode when the oxygen content in that atmosphere rises from zero percent to a certain percent (lower explosive limit) and this explosive mixture will continue until the mixture is so diluted with air that the volatiles within the mixture cannot support ignition (upper explosive limit). That concentration of oxygen in the atmosphere at the lowest possible percentage where the volatiles are capable of being ignited will be defined and used throughout the specifications as the "lower explosive limit" (L.E.L.). The "L.E.L." and "U.E.L." definitions used herein are reversed from their conventional meanings. Normally, combustibles are admitted to an air enclosed space until the L.E.L. is reached (typically about 4% combustibles) and the explosive mixture continues until the U.E.L. is reached (typically about 4% oxygen).
Other terms which tend to be confused in the art include "stoichiometric", "starved air" or "starved combustion" and "sub-stoichiometric". Stoichiometric is technically defined as an adjective characterized by being a portion of substances exactly right for a specific chemical reaction with no excess of any reactant or product. It is typically used in the burner art to mean that metered amounts of fuel and combustion air are supplied to the burner so that the fuel is completely combusted by the precise amount of air provided. "Starved air" means that the air is supplied at a rate which is less than stoichiometric when compared to that amount of oxygen required for stoichiometric combustion of the material. For purposes of this specification, the "starved air" mode will mean, arbitrarily, oxygen supplied at a rate equal to anywhere from 40-99 percent of the oxygen required to achieve stoichiometric combustion. "Sub-stoichiometric", as used herein, will mean significantly less oxygen than that applied in the starved air mode and will mean anywhere from 1-10 percent of the oxygen normally required to produce stoichiometric combustion. The terminology as thus defined is only of limited value because for certain exothermic reactions, combustion will occur even under the starved air and sub-stoichiometric conditions as a function of time. Nevertheless, the definitions are helpful to distinguish incinerator apparatus operated in a starved air mode and erroneously referred to as a pyrolyzer. For example see U.S. Pat. No. 4,649,834 to Heran (incorporated herein by reference) describing a water activated temperature control system for a pyrolyzer which, in fact, appears to be a furnace operating under starved air conditions. Reference may also be had to U.S. Pat. Nos. 4,474,121 and 4,517,906 to Lewis (incorporated herein by reference) which discuss starved air combustion in terms of stoichiometric relationships and identifies the pyrolysis misnomer applied to such processes.
Insofar as pyrolyzing processes are concerned, the present inventors have developed and perfected for batch type pyrolyzing furnaces a two step process comprising an endothermic first step where pyrolysis is conducted followed by an optional "burnout" or incineration second step which burns off any residual fixed carbon. The endothermic step is generally conducted at temperatures between 250.degree.-1400.degree. F. and the exothermic step is generally conducted at temperatures between 1400.degree.-2500.degree. F. This is the general process as conventionally practiced by the inventors and includes an afterburner for combusting the volatiles distilled from the waste during the pyrolyzing process. Heretofore, the incineration or burn-out step, being an exothermic reaction, was not controlled.
Within the published prior art, it is generally accepted that pyrolysis is defined as a two step process in the sense that the waste is pyrolyzed in the first step and the fumes or volatiles emitted from the waste are combusted in an afterburner in the second step. The reason for dividing the process into two steps is because the endothermic reaction can be controlled. In the starved air systems discussed above, the reactions which are both exothermic and endothermic cannot be controlled and thus the reason for many of the control schemes present in the art which are then necessary to prevent the waste material "taking off" and producing an explosive mixture.
Insofar as controllability is concerned with respect to the endothermic reactions, the inventors have developed for use in their pyrolyzing batch type furnaces, a concept defined herein as "signature heat profile". Insofar as is pertinent to the discussion of the prior art as practiced by the inventors, it is known to take a sample of a waste specimen, and pyrolyze the specimen at various temperatures while recording the weight loss of the specimen until volatilization is achieved in optimal processing times. The time-temperature profile thus obtained in the gravimetric furnace then becomes the "signature" which is programmed into the commercial pyrolyzing furnace for treating the waste. In this manner, batch pyrolyzing of complex, heterogeneous waste material (including many hazardous and toxic substances) containing competing reactions has now been achieved. The signature heat profile concept gradually evolved over a period of several years and it is still evolving. Heretofore, the signature heat profile concept has only been applied in batch furnace processes.
Outside of signature profiling, batch furnaces in the prior art simply fired or heated the work to some upper limit temperature and monitored the gases, not only in the furnace chamber but also downstream of the burner. When the process started running out of control, the heat was initially reduced and, if this was not sufficient, a water spray was introduced. In shaft and sliding bed continuous furnaces, a further variable, the rate of feed was controlled. Further, in shaft and sliding bed furnaces, the waste, after treatment, was simply examined and then temperature and rate was varied until an acceptable end product was produced. None of the control systems, with the exception of signature profiling, is generally adequate for commercial installations involving complex, heterogeneous waste materials.
With respect to the various apparatuses used to pyrolyze waste, a fundamental distinction is made between batch type pyrolyzing furnaces and continuous type pyrolyzing furnaces. The pyrolyzing furnaces for all intents and purposes are equivalent in structure or mode of operation to those used in the industrial furnace art. Batch type furnaces are essentially box type furnaces and are distinguished by the types of drive mechanisms used to transfer containerized waste into and out of the furnace. Batch furnaces are thus further defined as "roller hearth" where the containerized feed enters the furnace at one end and is discharged at the other end and rollers, either free or driven, move the load into and out of the furnace. "Roller rail" batch furnaces use, as the name indicates, rollers riding on a rail with charge and discharge fixtures external to the furnace to push or pull the work into and out of the furnace. Finally, "car bottom" furnaces are basically cars containing the waste riding on a rail which are either then lifted into the furnace chamber or the furnace chamber is dropped onto the car to define the furnace enclosure. Waste pyrolyzing has been successfully practiced in batch furnaces. The process has achieved limited commercial success in continuous or semi-continuous furnaces which usually involve installations dedicated to processing only a few types of waste materials.
Insofar as continuous furnaces are concerned, there are essentially three different types of furnaces. One of the most common is a rotary kiln pyrolyzer which comprises essentially a horizontal rotating cylinder with the load to be heated tumbled on the inside. Transfer of the waste, usually in the form of loose solids or sludges, can be assisted by angling the axis of rotation of the kiln a few degrees so that the charge end is higher than the discharge end. In addition, a helical auger is typically installed on the kiln's inside diameter transforming the kiln into a sort of Archimedes' screw. Another classification of continuous furnace includes vertical shaft and sliding bed furnaces where granular material is fed at the top of a cylindrical shaft and the material discharged at the bottom. Various bustle and tuyere arrangements are employed to treat the load falling through the furnace as a continuous fluidized bed. One of the stages or zones in the furnace usually employs a starved air if not a pyrolyzing zone. A third type of continuous furnace, which has experienced some commercial success, is defined as a rotary hearth which is normally used for processing of sludges or granular solids. It has a doughnut shaped hearth rotating in a stationary furnace chamber and waste material is continuously fed in a cold zone. A stationary spreader distributes the feed material uniformly in a radial direction as the hearth rotates under the spreader. The feed is passed through a hot zone while the solid residue is continuously discharged by a screw conveyor. Dropped arches in the hearth separate the hot and the cold zone. Because there is some similarity to the pyrolyzing furnaces thus described to the furnaces used in the industrial heat treat art, it should be noted that dropped arches are conventionally used in pusher slab reheat furnaces and the like employed in the steel mill industry to define zones wherein atmosphere and temperature of the work can be controlled for heat treat purposes. A barrier free rotary hearth has been achieved by the means of free standing jets as shown in assignee's U.S. Pat. No. 3,819,323 (incorporated herein by reference). Such jets are not suitable for establishing zones in a pyrolysis furnace where granular or loose material is to be pyrolyzed. None of the furnace arrangements described permit continuous or, more precisely, semi-continuous treatment of containerized wastes. The continuous furnaces can treat only loose bulk material (which for certain waste is not feasible) and it is very difficult, if not impossible, for certain wastes to accurately control a zone of a moving bed which can have varying densities, uneven temperatures, etc.
The furnace chamber where the waste is pyrolyzed is, generally, indirectly heated. Either electrically heated or fuel fired radiant tubes are used or any one of a number of schemes using the afterburner heat can be employed to indirectly heat the waste in the furnace chamber. It is also conventional to use direct fired burners to heat the waste in the furnace chamber. The burners are operated at stoichiometric or slightly less than stoichiometric conditions. In such arrangements which include all the batch furnaces, the rotary kiln and the rotary hearth, convective heat transfer is not significant. The waste is generally heated for the most part by conduction and radiation. This increases the time required for processing the material. Only in the vertical shaft and sliding bed furnaces does convective heating predominate. However, as noted above, such furnaces have inherent problems limiting their application. Obviously, the time for the pyrolyzing process is materially enhanced to the extent that convective heat transfer can be utilized, provided the endothermic reaction can be controlled.
Insofar as the end material after pyrolysis is concerned, it has always been known that the char recovered could function as a useful end product, i.e. fertilizer. It has also been known that pyrolysis can be used to recover, for recycling purposes, metal. For example, the Heran patent noted above uses starved air to burn off polymeric materials from metal parts so that the metal, i.e. copper in armatures, staters, etc., can be recovered. In U.S. Pat. No. 3,780,676 to Hazzard (incorporated herein by reference), a rotary kiln pyrolyzer is used to recover the aluminum or gold foil or silver found in circuit boards or photographic film.
As noted in the Hazzard patent, using starved air, such as in the Heran patent, generates metal oxides which then hinders the ability of the process to recover all the metal. However, simply conducting the endothermic step as defined in the Hazzard patent leaves "fixed carbon", a carbon residue or polymer embedded on the metal's surface. If the fixed carbon is simply burned off in the conventional incineration sense, the metal oxide problem is present. The presence of fixed carbon on the reclaimed metal may or may not present a problem in the metal recycling process depending upon the particular process which uses the reclaimed metal. However, the fixed carbon will always be a problem if a non-metallic heterogeneous composite material having organic material is present. With the exception of the metallic reclamation processes discussed, which are not perfected for reasons noted, there has not been any processes which can successfully and efficiently thermally reclaim heterogeneous metallic or non-metallic composites having organic compound(s) in a substantially pure form. When used throughout the specification and claims, the term "substantially pure" means fit for reclamation which, depending on the reclamation process for the base material, may or may not mean the base material in its elemental pure form.